The first ballots will be cast in the Labour leadership election in a contest that has been turned upside down by the surprise frontrunner – the London MP and anti-war campaigner Jeremy Corbyn.

Against a backdrop of increasingly desperate interventions from senior party figures appealing for those registered to vote to back anyone but Corbyn, ballot papers are expected to start arriving through the letterboxes on Monday of the 450,000 people who have registered to take part – many in the wake of Corbyn’s breakthrough in the polls.

On Sunday Gordon Brown became the latest senior Labour figure to warn against choosing Corbyn as the party’s next leader, suggesting that the MP for Islington North could damage international relations by allying with Hezbollah, Hamas, Venezuela and Russia. The former prime minister did not refer to any of the candidates by name, but his 50-minute speech included thinly veiled warnings that Corbyn would make Labour a party of protest rather than one of government.

On Monday the Daily Telegraph reported on a “secret plot” in which Peter Mandelson attempted to convince Corbyn’s three rival candidates – Yvette Cooper, Andy Burnham and Liz Kendell – to pull out to annul the contest. It is understood that there have been some frantic discussions about whether to attempt to derail the process, but with ballot papers now on their way it appears there is no going back.

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On Monday, Burnham will reach out to Corbyn and his supporters in an appeal for party unity as the leadership election threatens to tear Labour apart.

The shadow health secretary will say it would be “unforgivable” if infighting after the result is announced on 12 September prevented Labour standing up to the Tories. The appeal, in a speech in Manchester, follows Brown’s passionate call for Labour not to become a “party of protest” instead of a credible electoral force.

The former prime minister told party members and invited journalists at the Royal Festival Hall in London that Labour must be “credible, radical, sustainable and electable to help people out of poverty” and that anger was not enough. In a clear reference to Corbyn, he said there was one camp whose own supporters did not even believe their candidate would win the next election.

Brown said he was heartbroken and the party was grieving after the general election defeat in May, but that it would be “even worse if we leave ourselves powerless to do anything about it”.

In the most explicit warning so far about Corbyn’s foreign policy, he said: “Don’t tell me that we can do much for the poor of the world if the alliances we favour most are with Hezbollah, Hamas, Chávez’s successor in Venezuela and Putin’s totalitarian Russia.” Corbyn has been criticised for describing representatives of Hamas and Hezbollah as friends, which he has said was a collective term rather than a sign that he agrees with their views. Corbyn has also hinted at being open to a closer relationship with Russia and wants to withdraw from Nato.

Brown staged his intervention after Corbyn became the surprise frontrunner in the contest with the backing of the two biggest trade unions, Unite and Unison. Until recently an obscure leftwing backbencher, Corbyn only managed to get on to the ballot after a number of Labour MPs lent him their nomination in order to encourage debate about the future of the party.

Brown’s speech follows interventions by Tony Blair, Alan Johnson, Jack Straw and Alastair Campbell who have warned that Corbyn would be electorally disastrous. None, however, has dented the leftwinger’s status as the favourite to win and pressure is now mounting on former leader Ed Miliband to make clear his views, given that they are more likely to be respected by the left of the party than those of politicians more closely associated with the Blair era.

Cooper’s team had been hoping to win the endorsement of Brown. Her husband, the former shadow chancellor Ed Balls, was one of his closest advisers, but the former prime minister chose not to back a single candidate.

As ballot papers start to arrive on the doormats of party members, Burnham and Cooper are now engaged in a bitter fight to position themselves as the candidate who can beat Corbyn on second preference votes. Both are refusing to stand aside despite pressure from MPs for anti-Corbyn sentiment to converge behind a single candidate.

A campaign source has told the Guardian that people in Kendall’s camp tried to get both her and Cooper to withdraw and rally round Burnham. But Cooper refused and Kendall was not willing to drop out on her own. After a speech on Thursday, Cooper told the audience it would not have been right for the two women to give up, leaving a field of two men.

Responding to Brown’s speech, a spokesman from Corbyn’s campaign said it “highlighted the need for a Labour party that stands for hope, that is credible, radical and electable – on which basis, the best candidate to vote for is Jeremy Corbyn”.

He added: “It is necessary to be credible, but credibility cannot mean an orthodoxy of austerity that chokes off recovery. Instead we need a Labour party that stands for growth, investment and innovation across the whole country.

“Jeremy Corbyn’s clear plans for growth-led recovery rather than austerity mark him out as the candidate offering hope and drawing in thousands of new people in the process. Polls vary, but most have shown that Jeremy Corbyn is the candidate most likely to engage with voters beyond Labour’s existing supporters.”

Brown’s decision to break his silence on the contest drew a divided reaction from his party. Clive Lewis, a new Labour MP and Corbyn supporter, tweeted that Brown was not qualified to lecture on economic credibility.

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This in turn drew fire from other Labour MPs, with the shadow Treasury minister, Alison McGovern, replying: “I’m a ‘play the ball, not the person’ sort. But this is a joke. [Gordon Brown] helped millions to better life. And this guy? Not.”

The other three leadership candidates initially held back from criticising Corbyn over fears that they might be seen as ganging up on the outsider, but they have now all suggested that he risks creating turmoil in the party.

In an interview with the Sunday People, Burnham claimed that electing Corbyn would drag Labour back to the infighting of the 1980s. “I’m the only person in this race who can beat Jeremy,” he said. “In the 80s, we started fighting each other and left the way clear for Margaret Thatcher to bulldoze her way through Labour communities. I’m not going to let that happen this time.”

After a week of intense attacks from senior Labour figures about his credibility as a leader, Corbyn sought to calm fears that he would have an anti-business agenda by setting out plans to support entrepreneurs and small traders. He told the Observer: “The current government seems to think ‘pro-business’ means giving a green light to corporate tax avoiders and private monopolies. I will stand up for small businesses, independent entrepreneurs and the growing number of enterprises that want to cooperate and innovate for the public good.”

Corbyn has dismissed criticism of his suitability as a leader by saying that he does not do personal attacks or respond to abuse.

Jeremy Corbyn’s supporters

Dave Ward, CWU general secretary

“We think that it is time for a change for Labour. The grip of the Blairites and individuals like Peter Mandelson must now be loosened once and for all. There is a virus within the Labour party, and Jeremy Corbyn is the antidote.”

Mick Cash, RMT general secretary

“We believe that with Jeremy as leader, Labour would attract back much of the working-class support it has lost, and that his campaign enables us to show the popularity and importance of policies such as public ownership of public transport, repeal of the anti-union laws and opposition to austerity.”

Michael Meacher, Labour MP for Oldham West and Royton

“The reason that Jeremy Corbyn is so popular, and could actually win this contest, is that he uniquely stands for making a clean break with Tory policies, above all by advocating growth as the way to pay down the deficit, not austerity which is being used by the Tories, with the complicity of the Blairites, to destroy the whole of the post-war social democratic settlement.”

John McDonnell, Labour MP for Hayes and Harlington

“As people wake up to the prospect of Jeremy Corbyn actually being able to win the Labour leadership, the reaction has become increasingly hysterical, especially from elements of the Labour establishment ... A small band of shadow cabinet members have lined up to refuse to serve in posts they haven’t even been offered, on the basis of objection to economic policies they clearly haven’t read.”

Stella Christou, a 23-year-old Greek graduate

“New parties and new figures are emerging throughout Europe and now it is happening here. They are shaking the foundations of traditional social democracy. Jeremy Corbyn is bringing together the disaffected, demoralised and apolitical people.”

Jeremy Corbyn’s detractors

Tony Blair

“The party is walking eyes shut, arms outstretched over the cliff’s edge to the jagged rocks below. This is not a moment to refrain from disturbing the serenity of the walk on the basis it causes ‘disunity’. It is a moment for a rugby tackle, if that were possible.”

Peter Mandelson

“Those of us who stayed and fought to save the Labour party in the 1980s will be experiencing a growing sense of deja vu. The last five years have left us with a terrible legacy to overcome with the existence of the Labour party as an effective electoral force now at stake.”

Alan Johnson

“Jeremy’s ... been cheerfully disloyal to every Labour leader he’s ever served under. That’s fine so long as members understand that it’s the loyalty and discipline of the rest of us that created the NHS, the Open University ... Let’s end the madness.”

Jack Straw

“I know Jeremy and I know he simply could not do this job. It’s very serious for anybody to try be a leader of the Labour party without having the confidence of the parliamentary party, the Labour MPs. He plainly hasn’t got that.”

Alastair Campbell

“Whatever the niceness and the current warm glow, Corbyn will be a leader of the hard left, for the hard left, and espousing both general politics and specific positions that the public just are not going to accept in many of the seats that Labour is going to have to win to get back in power.”